1. Field of the Invention
The present invention relates to a non-cyanide electrolytic gold plating solution, and particularly relates to a gold plating solution excellent in its solution stability and capable of gold plating treatment with good plating properties, and a gold plating method using the solution.
2. Description of the Prior Art
Gold plating has been used not only for ornamental purposes or dinnerware, for example, for a long time but also in a wide range of industrial applications such as electronic and electric components and acoustic equipment components because of its excellent electrical characteristics.
Conventionally, most gold plating solutions were cyanide baths containing toxic gold potassium cyanide. However, the demand for a non-cyanide gold plating solution has recently been increasing due to problems concerning work-site safety or effluent treatment and to a problem concerning attacking a resist of a semiconductor component. Therefore, various kinds of non-cyanide gold platings have been suggested.
For example, many non-cyanide gold plating solutions use Na3Au(SO3)2 as a gold salt. However, in a gold plating bath which uses Na3Au(SO3)2, sulfite ions in the solution are unstable and easily oxidized by oxygen in the solution and by oxygen in the air, so that the concentration of the sulfite ions are spontaneously decreased. Consequently, the stability of gold complexes in the gold plating solution becomes lower and gold may precipitate. In addition, with the above described phenomena, a problem of inducing changes in plating properties would occur.
The inventors have attempted, to provide a non-cyanide electrolytic gold plating solution excellent in its solution stability and a gold plating method using the solution.
The inventors have made intensive studies to develop a non-cyanide electrolytic gold plating solution which is practically excellent, and have now found that the gold plating solution described below exhibits extremely outstanding performance.
The present invention is a non-cyanide electrolytic gold plating solution containing a trivalent gold compound which is a gold hydroxide salt and/or chloroaurate salt, a chelating agent which is a hydantoin compound selected from imidazolidinedione, 5,5-dimethylhydantoin, and hydantoic acid, a buffer, and a conductive salt, wherein the concentration of the gold in the gold plating solution is 0.5 to 30 g/L, the concentration of the chelating agent in the gold plating solution is 0.1 to 2.5 M/L, and pH is 5.0 to 10.0.
The gold plating solution according to the present invention, containing a chelating agent of a hydantoin compound, is excellent in its solution stability, and the gold precipitate will not be formed in the solution during the gold plating treatment. Further, the gold plating solution of the present invention can freely control a resultant appearance of the plating from a lustrous state to a lusterless state. Still further, a plating hardness among other plating properties is about 100 HV immediately after the plating treatment and 70 HV or less after the annealing (300xc2x0 C., 300 min.).
When the chelating agent according to the present invention, that is, a hydantoin compound is used, the gold plating solution becomes extremely stable compared with other gold plating solutions which have conventionally been known. In other words, gold will not precipitate during the plating treatment. This is because the chelating agent is not reductive as in the case of sulfurous acid and has a property such that autolysis caused by pyrolysis or electrolysis hardly occurs. With the gold plating solution of the present invention which uses this chelating agent, it becomes possible to perform the plating treatment within a pH range around neutrality from weak acidity to weak alkalinity. Therefore, even when a photoresist (referred to as PR hereinafter) is coated on aplated substance, the gold plating treatment can be performed without dissolving the PR.
As a hydantoin compound which is the chelating agent used for the gold plating solution of the present invention, it is preferable to use any one of imidazolidinedione, 5,5-dimethylhydantoin, and hydantoic acid. This is because these compounds are excellent in capability of producing gold complexes and hardly cause autolysis resulted from pyrolysis or electrolysis.
In addition, it is preferable to use any of trivalent gold hydroxide salts and chloroaurate salts or a mixture thereof as the gold compound in the gold plating solution according to the present invention. In the gold plating solution according to the present invention, the gold may be supplied by the gold salts or gold complexes. A source of gold comprising any of the above described substances or a combination thereof has good compatibility with the above described chelating agent, so that the excellent solution stability can be obtained.
The concentration of the gold in the gold plating solution according to the present invention depends on an amount of the chelating agent contained, but is preferably in a range of 0.5 to 30 g/L. This is because electrodeposition will not occur without application of voltages of 3v or more when the concentration is less than 0.5 g/L, whereas a visual failure will occur when the gold concentration is above 30 g/L even when the concentration of the chelating agent is increased up to its upper limit. The concentration of the chelating agent, taking the above described gold concentrations into consideration, is preferably in a range of 0.1 to 2.5 M/L. This is because the chelating agent does not work as an effective complexing agent when its concentration of the chelating agent is less than 0.1 M/L, whereas salting-out will easily occur depending on the pH of the solution when the concentration of the chelating agent is above 2.5 M/L. Therefore, it can be said that the gold plating solution should preferably have a composition of concentration values within the above described ranges, in order to make the solution to be practical in gold plating operations.
As the conductive salt in the gold plating solution according to the present invention, it is preferable to use any one or more of hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, and salts thereof. When using the conductive salt comprising any one of the above described substances or a combination thereof, the solution stability of the gold plating solution according to the present invention becomes extremely excellent.
Further, as the buffer contained in the gold plating solution according to the present invention, it is preferable to use any one or more of boric acid, succinic acid, phthalic acid, tartaric acid, citric acid, phosphoric acid, and salts thereof. Using the buffer comprising any one of the above described substances or a combination thereof prevents the pH of the gold plating solution according to the present invention from largely fluctuating, and facilitates keeping its pH around neutrality from weak acidity to weak alkalinity (pH of about 5.0 to 10.0).
When the above described conductive salt and the buffer are contained in the.gold plating solution according to the present invention, preferably the concentration of the conductive salt should be 0.05 to 2.45 M/L, the concentration of the buffer should be 0.05 to 2.45 M/L, and the total concentration of the conductive salt and the buffer should be in a range of 0.1 to 2.5 M/L. When the concentration of the conductive salt is less than 0.05 M/L, a current efficiency becomes worse due to the reduction in its conductivity, and also a visual appearance of the plating tends to be deteriorated. In addition, when the concentration is above 2.45 M/L, the salting-out easily occurs depending on the pH value. On the other hand, the buffer does not exert its effect of stabilizing the pH when its concentration is less than 0.05 M/L, and the salting-out easily occurs depending on the pH value when the concentration is above 2.45 M/L. Still further, when the total concentration of the conductive salt and the buffer is 0.1 to 2.5 M/L, the gold plating solution according to the present invention becomes practically the most excellent as a whole. That is, the solution stability is improved, the current efficiency becomes higher, and the pH of the plating solution is prevented from largely fluctuating.
When the gold plating treatment is performed with the above described non-cyanide electrolytic gold plating solution according to the present invention, the electrolytic plating should preferably be conducted under the conditions of the pH of the plating solution of 5.0 to 10.0, a temperature of the solution of 20 to 70xc2x0 C., and a current density of 0.1 to 4.5 A/dm2.
In this case, the pH value of the gold plating solution becomes within a range of pH5.0 to 10.0 depending on the concentrations of the buffer and the conductive salt, and an abnormal appearance does not occur in the deposited gold plating as long as the pH value is within this range. Non-uniform appearance of the plating will occur when the pH becomes less than 5.0, whereas the PR will tend to be dissolved if the PR is coated on a substance to be plated when the pH becomes above 10.0.
The reason why the temperature of the gold plating solution should be set within a range of 20 to 70 xc2x0 C. is as follows. That is, when the temperature be comes less than 20xc2x0 C., the gold plating solution will not be useful because fluctuation in the plating treatment becomes substantially too large. On the other hand, when the temperature becomes above 70xc2x0 C., luster of the deposited gold plating will adversely be affected by the temperature and a life of the solution will abruptly decreases.
The range of the current density, 0.1 to 4.5 A/dm2, has been determined by confirming that the properties of the deposited gold plating become extremely good situations, taking the above described pH value of the gold plating solution, the solution temperature, and the gold concentration into consideration. The properties of the plating in this case are meant that overall capacity including appearance, adhesive property, leveling, hardness or the like.
According to the above described gold plating method using the gold plating solution according to the present invention, the hardness of the deposited gold plating is about 100 HV (vickers hardness, this is the same hereinafter) immediately after the plating treatment, and 70 HV or less after the annealing under the condition of 300xc2x0 C.xc3x9730 min. The gold plating having such a characteristic including the above described hardness is effective as a bump deposit for IC implementation. In addition, using the gold plating solution according to the present invention, the solution becomes excellent in its long-term stability and also possible to be used for a long time.